參數(shù)資料
型號(hào)： AN-22
英文描述： OBSOLETE when inventory is depleted. 10% tolerance no l
中文描述： 設(shè)計(jì)多路輸出電源的的TOPSwitch
文件頁(yè)數(shù)： 17/24頁(yè)
文件大?。?/td> 569K
代理商： AN-22
C
5/98
AN-22
17
Additional Tips
Following are some tips which can be considered and tested
where necessary to improve circuit performance.
Optocoupler Connection
In multiple output power supplies, the current for the
optocoupler LED is often supplied via the loop gain setting
resistor from an output other than the main feedback voltage.
In Figure 4, this connection of R1 is made to the 5 V winding.
This technique introduces some AC feedback from the 5 V
winding, which helps reduce variation on that output during
transient load conditions.
R1 and R2 may be connected to the 12 V output instead of the
5 V output (with their values changed appropriately). Ripple
current from this output has a path to the TL431 reference pin
via R1 and C9. This type of connection, however, will often
introduce loop instability with very light loads on the 12 V
output. The reason is that the 12 V output is subject to peak
charging from energy in leakage inductance as its load
approaches zero. Peak charging effectively uncouples the
output so that it is no longer related to the 5 V output by the
turns ratio. If instability is observed during light or no load
conditions on the 12 V output, two options are available:
1. The optocoupler LED should be supplied from the 5 V
winding (with the value of R1 selected to maintain
acceptable AC gain) or
2. A dummy or minimum load resistor can be added to the
12 V output to eliminate the effects of peak charging.
Dummy loads are usually added to improve regulation at
light loads. R2 is used for this purpose on the 5 V output
in Figure 4. R2 might be moved to the 12 V output if one
dummy load is sufficient to meet specifications. The
value of this resistor should be adjusted as necessary to
allow for the load range of a particular application.
Soft Start Circuitry
Soft start circuitry is often useful to avoid output voltage
overshoot during power supply turn on. This is achieved
simply by introducing a capacitor from the TL431 cathode to
anode as shown by C15 in Figure 10.
Note a discharge path is required for this capacitor to insure
the soft start function is reset when the output voltages decay
at turn off. This function is provided in Figure 10 by the
minimum load resistor R2.
When introducing soft start, it is useful to supply the
optocoupler LED from a higher voltage output, such as the
12 V rail in this case, since this will insure that C15 begins to
charge and provide the soft start function as soon as possible
after the power supply starts to operate. The issues of
minimum load on the higher voltage output, discussed above,
must be considered when doing this to insure loop stability
under all conditions.
Improving Regulation in Lightly Loaded Outputs
Some outputs, such as the 30 V output in this example, can
have very light loads even under maximum load conditions.
These are prone to peak charging, which can produce output
voltages much higher than expected by the turns ratio of the
transformer output. The degree of this peak charging is
strongly influenced by the loads on the other outputs.
The output in question can simply be clamped with a Zener
diode between the output and secondary return. However, a
lower cost and more efficient solution is to provide some low
pass filtering that will reject the short voltage spikes from
leakage inductance to prevent charging of the output capacitors.
The introduction of a resistor in series with D5 in Figure 4 will
provide this function. Values from 10 to 100 ohms should be
tested to determine the optimum. See R6 in Figure 1.
Negative Outputs
Negative outputs are often required in a system for
operational amplifiers or other analog circuitry. Two simple
configurations generally used to provide these outputs are
shown in Figure 11.
Figure 11(a) shows the most usual configuration, where the
direction of the output diode is reversed such that that diode's
cathode is connected to the transformer
’
s output pin. The other
end of the negative winding is connected to the common
secondary return using the same dot convention as the other
output windings. An alternative technique connects the anode
of the output diode to the return end of the winding with the
cathode connected to the common secondary return as shown
in Figure 11(b). The alternate, however, is not available with
stacked windings.
The calculation of the number of output turns is identical to that
for positive outputs, and the same transformer construction
techniques are used to optimize cross regulation. Since
negative outputs are often lightly loaded, the techniques to
improve regulation in lightly loaded outputs detailed above are
often useful. Alternatively, the output can simply be post
regulated with a linear regulator.
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